A Defence spokesperson said that it has since reviewed its ICT systems and added new safeguards in the wake of the incident.

Defence declined to comment on the specific security safeguards it had introduced.

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The spokesperson said its graduates were robustly vetted by the Australian Government Security Vetting Agency and undertook training in classified document handling, code of conduct, and security and social media awareness.

But, now retired Lieutenant General Peter Leahy, who was chief of army between 2002 and 2008, has warned that vetting of staff can only do so much to ensure they will not spread government secrets illegally.

"They can't be stopped, and they can't be stopped because sadly we can't rely on the integrity of individuals across the board," he said.

While not commenting on any specific case, Professor Leahy, now the director of the University of Canberra's National Security Institute, said it was difficult to prevent leaks, saying a lot of faith was placed in the integrity of staff.

"If someone chooses to go against the commitment and oath that they've made, you just can't stop them, it's their choice," he said.

Scerba, then a 21-year-old Defence graduate, allegedly accessed the Secret Defence Security Network to download the DIO intelligence assessment in October 2012.

He is accused of posting the first two pages to online image-sharing forum 4chan. The first post was said to have included the comment "Julian Assange is my hero".

Later comments are said to have allowed the Australian Federal Police to track an IP address to Scerba's home in Canberra's south. They forensically examined his study computer, which had no password, and found traces of the DIO report, prosecutors said. The 4chan website was one of the most visited, according to court documents.

Commonwealth prosecutors have alleged the document directly revealed the identity of intelligence sources, gathering methods, and classified aspects of strategic partnerships between Australia and foreign countries.

The document was meant for the "Five Eyes" intelligence alliance involving Australia and its top allies.

Scerba possessed a secret, or negative vetting level one, clearance, and had been with the department for about eight months.

Mr Leahy said that vetting could only provide so much assurance that staff would do the right thing.

"The vetting process is extensive and expensive, but if someone chooses to lie during the vetting process and can cover up their lie, or even after the vetting process, chooses to act in an illegal manner, there's not much you can do about it," he said.